Prerequisite Knowledge on Programming the Atari VCS in Assembly

How a Television Works

Mon, 01 Jan 0001 00:00:00 +0000

How a Television Works#

Before the VCS makes sense, the television has to. Every concept in this book — the frame loop, sync signals, “Racing the Beam,” the cycle budget — is a direct consequence of how a cathode-ray-tube TV paints a picture. You don’t need a broadcast-engineering course; you need the handful of facts the hardware forces you to care about.

The beam paints one line at a time#

A CRT television draws its image with a single electron beam that sweeps across the screen from behind. It moves in a fixed pattern:

Reading 6502 Assembly

Mon, 01 Jan 0001 00:00:00 +0000

Reading 6502 Assembly#

Every line of code in this book is 6502 assembly language — human-readable names for the numeric machine code the 6507 actually executes. DASM translates it into the bytes that go on the cartridge. You don’t need to write it fluently yet, but you do need to read it, because every concept here is demonstrated in it. This page is just enough to follow along; the CPU’s full instruction set and addressing modes are the 6502 Basics chapter.

Thinking in Bits

Mon, 01 Jan 0001 00:00:00 +0000

Thinking in Bits#

On the VCS, almost every hardware register is a bag of individual switches — one bit turns on playfield reflection, one bit is the fire button, a group of bits picks a sound waveform. There is no high-level API; you set and clear bits directly. This page is the bit literacy the rest of the book assumes. Its companion, The Memory-Mapped Interface, covers where those bits live and how you reach them.

Numbers & Arithmetic

Mon, 01 Jan 0001 00:00:00 +0000

Numbers & Arithmetic#

Thinking in Bits covered a byte as a pattern of switches. This page is about a byte as a number — and the surprising limits of doing math on a 1977 processor. The big ones: there are only 256 possible values, negative numbers are a convention rather than a feature, and the 6502 cannot multiply or divide at all.

A byte is 0–255 — or −128 to 127#

An 8-bit byte holds 256 distinct values. Read as unsigned, that’s 0 to 255. But the CPU has no separate “signed” type; instead, by convention, the top bit is treated as a sign using two’s complement:

The Memory-Mapped Interface

Mon, 01 Jan 0001 00:00:00 +0000

The Memory-Mapped Interface#

On the VCS you talk to hardware by reading and writing memory addresses. The chips that make sound, draw pixels, and read the joystick live in the same address space as RAM, so a sta to the right address isn’t storing data — it’s operating a chip. This page assumes the bit literacy from Thinking in Bits; here we cover where those bits go.

Registers are just addresses#

The 6502/6507 has exactly one kind of “talk to the outside world” instruction — the same loads and stores it uses for RAM. There is no separate I/O instruction. Instead, the TIA (graphics/sound), the RIOT (RAM, timer, and joystick/switch ports), and the cartridge ROM are all wired to respond to specific addresses. This is memory-mapped I/O: the chip behind an address decides what reading or writing it actually does.